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EVOLUTIONARY HOMOLOGY IN THE FIN-TO-LIMB TRANSITION: EVALUATING THE MORPHOLOGY OF FORAMINA IN A LATE DEVONIAN HUMERUS FROM THE CATSKILL FORMATION, CLINTON COUNTY, PENNSYLVANIA Project Public
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  ID Part Object Taxonomy Type Date Added Publication Status  
000346602 Humerus ansp:vp:21350 Mesh 2021-02-26 Open Download  
000165319 Humerus ansp:vp:21350 Ct image series 2020-10-14 Restricted Download  
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DESCRIPTION

The shift from life in the water to life on land is regarded as a
key transition in vertebrate evolution. This transition in tetrapodomorph vertebrates is marked by a number of morphological
shifts, including, but not limited to, the evolution of a neck
(Daeschler et al., 2006, Shubin et al., 2015), distinct digits
(Coates and Clack, 1990, Boisvert et al., 2008), and weightbearing limbs (Coates and Clack, 1990, Niedzwiedzki et al., ́
2010, Clack, 2012, Pierce et al., 2012, 2013, Hohn-Schulte et al.,
2013, Molnar et al., 2018). The forelimb, specifically the
humerus, has been highlighted in this transition because of its
reorientation in the evolution of a weight-bearing limb (Coates
and Clack, 1990, Lebedev and Coates, 1995, Coates, 1996,
Shubin et al., 2004, 2006, Boisvert, 2009, Sanchez et al., 2014,
Daeschler and Downs, 2018, Downs et al., 2018). Early in tetrapodomorph evolution, a novel humeral feature emerged: the ventral
ridge (Shubin et al., 2004, Clack, 2012, Sanchez et al., 2014,
Downs et al., 2018). The ventral ridge is an ossification resulting
from the relocation of humeral musculature (Clack, 2012,
Molnar et al., 2018), and it commonly has many foramina in its
proximal and/or distal surfaces (Shubin et al., 2004, 2006, Boisvert, 2009, Daeschler and Downs, 2018, Downs et al., 2018,
Kamska et al., 2018). However, the morphology, function, and
evolutionary history of these foramina have not been explored
previously.
ANSP 21350 (Shubin et al., 2004, Daeschler et al., 2009), an
isolated left humerus from the Late Devonian of Pennsylvania,
preserves both plesiomorphic and derived characteristics in the
fin-to-limb transition (Ahlberg, 2011, Clack, 2012). The taxonomic and phylogenetic position of ANSP 21350 remains uncertain (Daeschler et al., 2009, Ahlberg, 2011, Clack, 2012),
although its morphology remains unique and informative, particularly with respect to that of the ventral ridge. The ventral
ridge in ANSP 21350 preserves five distinct foramina in conjunction with a tripartite entepicondylar foramen (Shubin
et al., 2004). However as with other tetrapodomorphs, the
internal morphology of the ventral ridge and the extent to
which foramina anastomose in ANSP 21350 remains a
mystery. Here, we reconstruct the internal morphology of
ANSP 21350 using micro-computed tomography (µCT) scan
data, and we present an evolutionary pathway for the ventral
ridge foramina that incorporates the existing tetrapodomorph
fossil record.

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